The invention relates to a device for scanning an optical record carrier comprising optically detectable marks along a track, which device comprises a radiation source for emitting a radiation beam, an objective system for guiding the radiation beam to the record carrier, and a radiation-sensitive detection system for receiving radiation from the record carrier, the detection system comprising at least two detectors on both sides of a dividing line, each detector having an output for providing a detector signal, and an electronic circuit for forming a focus error signal from the detector signals.
A device according to the preamble is known from U.S. Pat. No. 4,010,317 (PHN 6 224). The device comprises an objective lens for focusing the radiation beam onto an information layer of the record carrier in which information is stored in the form of marks. The radiation beam reflected from the record carrier is incident on two detectors of the device are arranged in the far field of the information layer. The phase difference between output signals of the detectors is used to form a focus error signal. This focus error signal is used for controlling the axial position of the objective lens. A disadvantage of the known device is that of the focus error signal depends on the type of record carrier.
The phrase xe2x80x9cthe detectors are arranged in the far field of the information layerxe2x80x9d is to be understood to mean that these detectors are located in a plane where the various diffraction orders of the beam from the information layer are sufficiently separated, i.e. in a plane which is disposed sufficiently far from the image of the information layer formed by the objective lens.
It is an object of the invention to provide a device for scanning a surface, which forms an accurate measure of the focus error, independent of the type of record carrier.
This object is met by a device as described in the preamble, which device is characterized according to the invention in that the electronic circuit is arranged for determining a time or phase difference between corresponding parts of the detector signals relating to passage of the radiation beam over patterns of the marks having a spatial frequency in a first range and in a different, non-overlapping second range, and forming the focus error signal in dependence on the time or phase difference. When the focus error signal is derived from the difference of two measurements, one in each range, the signal will be less affected by differences between different types of record carrier.
The marks used for measuring the time or phase difference are divided into two groups, one group preferably having a spatial frequency in a range from 0.14 to 0.2 times the cut-off frequency of the objective system, the other group in a range from 0.35 to 0.5 times the cut-off frequency. The focus error signal derived from these groups is relatively insensitive to the shape, size and phase depth of the marks, making the focus error signal independent of the type of record carrier.
The invention further relates to a method for determining the focus error from a time or phase difference measurement.
The invention also relates to a record carrier having the above patterns of marks located at specified parts of the tracks.